Growing wave electron discharge device



C. W. HANSELL GROWING WAVE ELECTRON DISCHARGE DEVICE Filed March 26, 1949 July 20, 1954 2 Sheets-Sheet 1 wkiwmmuw INVENTOR NWQYRM. I 1 I I I I I I n 1 1 Q: Q 55$ 5% 5% T-i E fi wa R \vm. Lisa T a 158 Eg m R a Q 5% C. W. HANSELL GROWING WAVE ELECTRON DISCHARGE DEVICE Filed March 26, 1949 July 20, 1954 2 Sheets-Sheet 2 INVENTOR CLARENCE W- HANSELI. Y a M WORQ Patented July 20, 1954 UNITED STATS TEN-T OFFKIE DEVIC Clarence W. Hansell, Port Jefierson, N. 51., assignor to Radio Corporation of America, a corporation of Delaware Application March 26, 1949, Serial No.- 83,697

( c1. s'i5- c 9 Claims. 1 This invention relates to electron devices and systems utilizing the same, and more particularly; to a stable, high gain, growing wave amplifier for operation at high frequencies.

A recently developed electron beam tube, called a traveling wave tube, is described in published references listed below.

Field Theory of Traveling-Wave Tubes, by L. J. Chu and J. D. Jackson, Proceedings I. R. E., vol. 36, No. 7, July 1948 Traveling-Wave Tubes, by J. R. Pierce and Lester M. Field, Proceedings I. R. E., vol. 35, No. 2, February 1947 Theory of the Beam-Type Traveling-Wave llube, by J. R. Pierce, Proceedin'gs'l. R. E., vol. 35, No. 2, February 1947 The Traveling-Wave Tube as Amplifier at Microwaves, by Rudolf Kempfner, Proceed ingsI. R. B, vol. 35, No. 2, February 1947 Small-Signal Analysis of Traveling-Wave Tube, by C. Shulman and M. S. He'ag'y, vol. 8, No. 4, December 1947, RCA Review In this traveling wave type of tube electromagnetic waves are propagated along an elongated conducting helix of such diameter and pitch that the axial velocity of the waves along the helix is reduced to a fraction, say one-tenth, of the velocity of light, and an electron beamis projected along the helix, either inside or outside, at a velocity approximately equal to, but somewhat greater than, the velocity of the waves along the axis of the helix. Under such conditions, the electron beam and the waves interact with each other and cause the amplitude of the waves to increase as the waves travel along the helix, and hence, produce amplification oi the waves.

The traveling wave tube has several objectionable features, the principal one being that there is a direct feedback coupling path from the output to the input ends of the tube by means of the wire helix directly connecting the terminals. Any wave reflections at any frequency at which the amplification is sufficient can cause undesirable spurious oscillations. Since reflections are impossible to suppress at all frequencies for which the tube has amplification, regeneration is always present. In order to keep regeneration within reasonable bounds in such tubes, it has been necessary to use circuit attenuation along the helix or in some region between the ends of the helix, which reduced the gain and power output,.and the efliciency of the tubes. For best results, the wire helix must be precisely'de'signed 2. to obtain the best relation between the electron velocity and the velocity of electromagnetic waves on the helix, throughout the wholeleng'th of the helix. This is very difficult to accomplish and, furthermore, the velocity of the electron beam varies with the amplitude of the waves which derive their energy from the kinetic eriorgy of the electrons so that, in any case, correct velocity relationships can never be obtained simultaneously over a range of amplitudes and operating frequencies.

It occurred to me that a simpler, cheaper, and better tube could be designed, which would elim'i n'ate or at least mitigate the objectionable features of the traveling wave tube, if the wave growth were made to take place in moving electron streams without any associated electromagnetic circuits except at the input and output terminals of the streams. With suitable input and output coupling means the tube could receive waves at one energy level and transmit them again without change of frequency at a greatly increased energy level.

In accordance with my invention, therefore, the conducting helix in a traveling wave tube is replaced by a second electron beam having an average electron velocity different from and lower than that of the first electron beam. The result is a growing wave' amplifier tube in which the principal amplification may be all-electronic, and in which there is no substantial feedback of energy from output to input terminals to cause undesired frequency selectivity effects, instabili ty, oscillation, and waste or power gain. Dueto elimination of the wire helix, there is substantially no direct coupling from the output to'thc input circuit. Because of the forward motion of both electron beams" there should not be any reflected backward wave such as produces feedback in traveling wave tubes. The absence of substantial feedback in the tubes renders unnecessary the use of circuit attenuation in electrical circuits paralleling the beam, and hence, eliminates theloss of gain and power output clue tosuch attenuation. The problem of designing a helix to match-beamand wave velocities over a long distance is-not' present in my growing wave tube and it does not matter much if the amplitude of the waves varies, so far as velocity re lationships are concerned.

The principal'o'bje'ct of my invention ist'o provide an improved and practical electron discharge device'o'r tube of the" beam type. More specifically, it is an object" of my invention to provide a beam tube utilizin a lurality of'electron beams in space charge coupling relationship with each other, particularly adapted for use as an amplifier. A further object is to provide a growing wave amplifier tube with no substantial internal feedback. Another object is to provide a wide-band amplifier particularly useful for amplifying frequency-modulated signals. Still another object is to provide an amplifier of high power gain which is capable of operation at very high frequencies such as, for example, 100 to 30,000 megacycles per second. Another object is to provide an amplifier with a very broad frequency band so that each specific design or" tube may be used at any portion of a very broad range of frequencies, frequency selectivity being applied externally to the tube. Furthermore, each tube is capable of handling very broad frequency pass bands such as are needed for such services as television radio relaying from city to city and for television broadcasting transmitters and receivers.

Consider two electron streams having different electron velocities traversing either the same or closely adjacent electron paths. If a small wave, in the form of small electron bunching, is started at the input end of either or both of the streams this wave or bunching will tend to grow, exponentially at first, towards an ultimate limiting value, because of the space charge interaction between the two electron streams. Electrons in one stream as they change position relative to the moving electron bunches in the other stream will add energy to the other stream to increase the bunching. The waves, as they grow, derive their wave energy by forcing the two electron streams towards equal average velocities. The final result, after some optimum distance, determined by the electron velocities, densities and paths, as well as by the amplitude of the initial punching, can be a very large degree of bunching of the electrons of the two streams, accompanied by diminished difference in electron velocities.

In order to visualize why the use of electrons of difierent velocities can provide a regenerative growth of waves in an electron stream, one can irnagine himself moving along the stream at a velocity corresponding to the Wave velocity, or velocity of regions of increased and decreased electron density, which generally will be about equal to the mean electron velocity. The electrons would then appear to be moving in both directions with respect to the observer, but any bunching of electrons in the stream in the nature of a growing wave would appear to be stationary, in which case one would expect a growth of apparent stationary electron waves or bunches, with respect to time, if for any reason a small initial bunching gets started. By way of analogy the phenomenon is similar to vehicular traffic jams, which occur on the highways in bunches, when the vehicles are driven at different speeds.

Any increase in electron density at any point along the path of the streams results in a more negative potential at that point, and any increase in electron density results in a less negative potential. However, the velocities of electrons moving in either direction in the stream are varied or modulated by the potential diiierences along the stream. The velocities will be least where the potential is most negative, corresponding to positions where electrons are more numerous, and the velocities will be greatest where the potential is least negative and the electrons are less numerous. The variations in velocity, in

turn, cause the electron densities of each of the passing streams to be increased where the velocities are least, and decreased where the velocities are greatest. Consequently, any small initial electron bunching set up by waves in the elecron streams tends to increase and, if the tendency for growth of waves exceeds the tendency for waves to diminish, the net effect will be a growth of the waves with time. The fact that such wave growth does in fact take place has been demonstrated by experiment.

When the growth of waves, or of electron bunches takes place with passage of time, as electrons mov from input to output coupling circuits it will be obvious that they also grow with increasing distance from the input circuit.

One conception of the growing wave amplifier is that it is a special form of induction generator, analogous to a two or three-phase electromechanical generator, having condensers connected across the windings, in which the wave path does not close on itself, and in which waves traveling along two wave media are made to grow with passage of time and distance by giving one wave medium a physical velocity with respect to the other. Energy is transferred from one medium to the other in such manner as to tend to reduce the relative physical motion therebetween and to build up the energy of the waves. The waves move along with reduced velocity in one medium and increased velocity in the other but with increasing energy derived from the reaction between the two media.

Some readers might prefer to think of a mechanical analog in which two rubbing rubber belts are made to move along in the same direc tion but at different velocities. Waves introduced in the rubber belts then will modulate the frictional coupling between them and the variations in the coupling along the belts can cause growth of waves in both belts. The such belts can represent two electron streams of different velocities occupying closely adjacent paths. Better esults may be obtained by using a large number of thin belts of alternately higher and lower velocities just as better results are obtainea with well mixed electron streams of different electron velocities.

In my multiple electron stream amplifier there is no need to keep the electron streams separated, because there is an elastic, or reactive coupling between streams of different velocity even though they pass through the same space. This is because electrcns of one velocity can pass through space charge clouds of another velocity but are always held apart by elastic repulsive forces, equivalent to capaci ive reactive coupling between the streams. At the same time, since the moving electrons constitute electrical currents they also have a mutual magnetic couplin Experiments, so far, have indicated that best results are obtained when the two beams of different electron velocities are mixed as intimately as possible where it is desired that growth of waves take place.

The input wave energy to be amplified can be applied to either the faster or the slower stream (or part of a s ngle stream) or, preferably, to both streams together. However, maximum signal to noise ratio would require feeding the input power into a low current stream and then progressively adding current of higher and lower velocities to the stream to cause the wave to build up to a large final value.

In the case of two streams with two different velocities, when a section of coiled transmission line is used as an input coupling, it is possible to 5. so adjust the electron velocitie that one is above and one below the velocity of waves on the coil. In this case the electron stream with lower velocity tends to load and decrease the waves in the coil as they move along whereas the electron stream with higher velocity tends to increase the waves in the coil. These two effects can be made to balance so that the overall coupling between the electron streams and the coil is zero, or very low. In this case waves in the coil serve only to introduce a variable coupling from one stream to the other in rhythm with the waves in such a way as to cause growth of waves in the electron streams without growth or attenuation of the waves in the coil. For given electron currents this condition is expected to provide a nearly minimum introduction of noise as well as an absence of regeneration or degeneration in the input circuit. However, this matter has not been fully determined and deserves further investigation.

At the output end, when a coiled line output circuit is used, on the other hand, it is generally desirable that the mean electron velocity of the two streams, where they enter the output coil, be greater than the wave velocity on the coil, so that relatively large wave energy from the streams will be transferred to the output circuit. In many cases the Waves in the stream, as well as in the output coil, may be made to grow with dis tance along the output coil to increase the gain and power output. Since the velocities of the electrons will decrease as they deliver wave energy to the output coil, it will be desirable to subject the electrons to some compensating acceleration a they pass through the output coil, or to vary the pitch of the coil as described by my associate, N. E. Lindenblad, in his U. S. Patent No. 2,300,052, or a combination of both, particularly when relatively large power output is required.

The novel features of my invention are set forth with particularity in the appended claims, but the invention itself will best be understood by reference to the following detailed description taken in connection with the accompanying drawings, in which Fig. 1 is a view (not to scale) in longitudinal section of an electron discharge device constituting one embodiment of my invention; Fig. 2 is a transverse section view in the direction of the arrows on line 2-2 of Fig. 1;. Fig. 3 is a group of graphs used in the description of the invention; Fig. 4 is a view in longitudinal.

section of a modification of the electron discharge device illustrated in Figs. 1 and 2; and Fig. 5 is a transverse section view taken on line 5-5 of Fig. 4.

Referring to Figs. 1 and 2, the electron discharge device comprises a first cathode l having a heater 2, a second cathode 3 in the form of a concavering forming part of a spherical surface and having an insulated heater 4 embedded therein, a metal drift tube 5 and a collector electrode 1, spaced along the longitudinal axis of the device. The cathodes i and 3 are rigidly mounted relative to each other and the drift tube 5 by means comprising a cup-shaped dielectric member 9, dielectric ring. members I I, l3, l5, and IT and apertured metal discs or rings I9, 2!, 23, 25, and 21. The dielectric members and metal. rings are hermetically sealed together and the ring 2? is sealed to one end of the drift tube 5 in the location illustrated in Fig. 1 in order to provide a vacuum envelope surrounding the two cathodes;

The drift tube 5 serves as part of the envelope and the opposite'end thereof is'sealed tothefirst 6, of three metal rings 29, 3! and 33 between which are sealed two dielectric rings 35 and 3?, the ring 33 being sealed to the collector electrode 1, as illustrated, to complete the vacuum envelope of the device.

The rings I9 and 2| have apertures axially aligned with the cathode i and the aperture in the ring cathode 3. The inner diameters of the ring 25 and drift tube 5 are approximately equal to the outer diameter of the ring cathode 3. The inner diameters of the rings 29 and 3E and the collector electrode 7 are approximately equal. In some cases, where operating frequency is high and wavelengths in the beams are short, the apertures in rings 29 and 3| may be covered with metal screens having a large ratio of opening to projected surface of metal in the screens.

The input means, as illustrated for example, comprises a rectangular wave guide cavity resonator 39 which terminates in a closed end ll and is provided with aligned apertures 53 and 55. The guide 39 is mounted on the rings l9 and 2| so as to form therewith an input cavity resonator for velocity modulating the electron beam from cathode I in accordance with a high frequency signal. The resonator portion of the guide is separated electrically from the remaining portion by transverse wall t! having a coupling iris 49'. The output means illustrated comprises a rectangular wave guide cavity resonator 5i similar to the input wave guide 39 and mounted on therings 29-and 3| to form therewith an inductive output cavity resonator through which the combined electron streams pass to the collector elec trode I. As illustrated schematically in Fig. 1, the electron beams from the two cathodes l and 3 are accelerated to different average velocities by connecting the cathodes to different negative potentials relative to a high positive potential applied to the drift tube 5. The ring cathode 3' is held at the less negative (higher) potential, and hence, the velocity of the outer electron stream is less than that of the inner stream. The direct current density of the inner stream is de termined primarily by the potential difference between the cathode I and the ring it, while the direct current density of the outer stream is determined. primarily by the potential difference between. the cathode 3 and the ring 25 and drift tube 5.

The focussing action of the spherical surface of the cathode 3 and the spreading of the inner beam cause the two beams to mix together within the drift tube 5. Preferably, the beams should. be thoroughly mixed where growth of waves is to take place and where the launching resulting from the initial velocity modulation of the inner beam is a maximum.

To prevent too much undesirable spreading of the beams within the drift tube 5, an axial focusing magnetic field may be providedv along the drift space within the tube, by means of an electromagnet coil disposed around the tube 5, as shown in Fig. 1. In general, for the tube of Fig. 1, the function of the magnetic field is to prevent too much loss of beam current to the drift tube 5. The dotted lines shown in Fig; 1 between the two cathodes and the collector represent the approximate paths electrons would follow in a weak magnetic focusing field.

Fig. 3 illustrates graphically how bunching of the electrons in one stream causes launching in a second stream of different velocity. Curve (1 represents the bombing along the drift tube in one stream at a particular instant. The ordinate is proportional to the charge density, and the dotted line represents the average charge density. Assuming the streams to be moving to the right, in Fig. 3, curve In represents the axial forces exerted by the electric fields due to the bunched first stream on a faster stream also moving to the right mixed with or otherwise in coupling relation with the first stream. Curve c represents the distribution of electron velocities in the faster stream produced by the electric fields of the bunched first stream. Curve (1 represents the resultant distribution of charge in the faster stream produced by the velocity distribution shown in curve c. Since the bunches in the two streams occur at substantially the same locations the result is a growth of the initial bunching. The greater the initial bunching is, the greater the resultant rate of growth of bunching will be. The growth of bunching can take place regeneratively up to an ultimate limit where electrons no longer can pass either forward or backward over the region of peak negative space charge potential. The degree of bunching which might be reached for this limiting condition, where the slip or diiference in velocity between the two streams reaches zero, is dependent chiefly upon the ratio of difference to mean of the initial electron velocities of the two streams, near the cathodes. In general, in the type of tube illustrated in Fig. 1, the signal modulation of the electron density along the stream will not approach closely to 100 per cent but the increase in bunching and amplification from input to output circuit may be substantial.

A preferred or improved modification of the discharge device of Figs. 1 and 2 with which power gain up to about 60 decibels was obtained experimentally, is shown in Figs. 4 and 5. This device is basically similar to that of Figs. 1 and 2, utilizing a different kind of input and output means and greatly improved means for mixing the electron streams. The device comprises an elongated dielectric envelope 6! containing a first oxide coated cathode having a heater 55, an annular second cathode 6?, a metal drift tube E9, and a collector electrode ll, spaced along the longitudinal axis of the envelope.

The first cathode 53 is surrounded by a tubular beam forming and focusing electrode 33. The second cathode s1 is in the form of an electronpervious electrode, in the form of a perforated or mesh grid, made of electroformed material similar to that employed in gasoline strainers, mounted over the central aperture of a concave ring focusing electrode l5. I have not determined an optimum percentage of opening for the perforations in the second cathode but have obtained high gain in a tube with about 30 per cent opening in the form of many small square holes in a sheet of nickel upon which barium and strontium oxides were coated. This provided an intimate mixing of the two electron streams. An accelerating electrode ll is provided. The second cathode El is heated by electron bombardment from the first cathode and requires no special heater element, or heater element connection. The two cathodes and the focusing and accelerating electrodes are mounted in axial alignment by means of leads extending through a re-entrant portion E9 of the envelope 6! as shown.

The cathode end of the drift tube 63 is centered within the envelope by means of a plurality of radial rods iii attached to the tube. The other end of the tube 69 is supported by hermeti- 8 cally sealed, input and output coaxial lines 83 and attached to the tube and extending through a re-entrant portion 85 of the envelope.

The input and output coupling means comprise short sections 8'5' and 39 of coiled transmission line forming continuations of the center conductors of coaxial lines and 85 and terminating on the tube 6%, as shown in Fig. 41. The open ends of the coaxial lines 83 and t5 are covered by shields 33' and 35'.

In one model of a discharge device as constructed according to 4 and 5, each coil had 19 /2 turns, wound 26 per inch, 8.5 centimeter inside diameter, the distance between adjacent ends of the coils being 10 centimeters. An axial magnetic field of about 1900 to 1200 gauss was provided by a coil (not shown) similar to that shown in Fig. 1. Beam accelerating potentials for maximum observed gain were about 1960 and 1400 electron volts and the total current of both beams effective to produce amplification was about 59 milliarnperes.

To test the device, the tube potentials and currents were applied in pulses, using a 60 cycle power source, as indicated by the transformers 9!, 92, and 83, a 1006 ohm resistor 2d, a selenium rectifier $35, and a SIM type rectifier tube 95. Meter 11 indicated the initial electron emission curren from cathode 53. Meter 1'2 indicated the difference between current collected and emitted by cathode El. 13 indicated the current to electrode '87. ll indicated the current collected by the input and output helices 8'5 and 8?; and the drift tube 59. I5 indicated the collector current.

The output coaxial line 8-5 was coupled back to the input coaxial line $3 through variable attenuation in the form of a T junction ill and a fixed attenuator 98 consisting of a high loss transmission line type RG-Zl/U. The total attenuation in the external feedback circuit was varied up to about 55 to 60 decibels (db). The internal cold attenuation of the tube between output and input terminals was greater than 60 db, the limit of our measuring equipment, at 2080 megacycles. A coaxial line type of wavemeter was used to measure the frequency of waves in the feedback path. A crystal detector Ed and an oscilloscope were used to observe absence or presence and relative amplitudes of waves. The amplifier comprising the tube of Fig. 4, oscillated at about 2140 megacycles with as much as 60 db total attenuation in the feedback path. When the feedback path was open circuited there were no oscillations. On the oscilloscope the starting and stopping of oscillations showed a series of pulses while the potential was changing, the number of pulses being greatest when the feedback attenuation was least. As the feedback attenuation was increased up to 60 db the pulsed oscillations at lower potentials disappeared progressively until there was only one pulse of oscillation at potential peaks corresponding to 1960 and 1400 volts electron energies, indicating that the gain of the particular device was lower than the maximum at all potentials below the peak.

To determine whether the amplification obtained was actually due to the difference in ve locity of the electrons in the beams, rather than to a velocity modulation phenomenon, of the kind utilized in Klystron type amplifiers, the phase of the 60 cycle voltage E1 between the two cathodes was made with respect to the phase of the voltage E2 between the second cathode 61 and the accelerating electrode all which was connected to drift tube 69 as shown. With this re- 9 versed phase relationship the amplifier would not oscillate although there was no substantial change in peak electron current and electron velocity in the beam passing through input and output coils 81 and 89.

While I have indicated the preferred embodiments of my invention of which I am now aware and have also indicated specific applications to amplifiers and oscillators, it will be apparent that my invention is by no means limited to the exact forms illustrated or the uses indicated, but that many variations may be made in the particular structure used and the purpose for which it is employed without departing from the spirit of my invention as set forth in the appended claims.

I claim:

1. An electron discharge device comprising means for producing a first beam of electrons along a given beam path, means including a concave annular cathode coaxially surrounding said beam path for producing a convergent hollow second beam of electrons surrounding and intersecting said first beam, means for modulating at least one of said beams, and means for extracting electrical energy from said beams.

2. An electron discharge device according to claim 1, further comprising an elongated drift tube surrounding said beam path between said modulating means and said energy extracting means.

3. An electron discharge device comprising: means, including a first cathode, for producing a first beam of electrons along a given beam path; means, including a concave annular second cathode coaxially surrounding said beam path, for producing a convergent hollow second beam of electrons surrounding and intersecting said first beam, means for velocity modulating said first beam only, and means for extracting electrical energy from said beams.

4. An electron discharge device according to claim 3, wherein said velocity modulating means comprises a pair of apertured electrodes surrounding said beam path between said first and second cathodes, said electrodes being spaced from each other and from said cathodes.

5. An electron discharge device according to claim 4, further including a hollow waveguide coupled to said apertured electrodes.

6. An electron discharge device according to claim 3, wherein said velocity modulating means comprises a cavity resonator located between and spaced from said first and second cathodes and having aligned apertures through which said beam path lies.

7. An electron discharge device according to claim 3, further comprising an elongated drift tube surrounding said beam path between said velocity modulating means and said energy extracting means.

8. An electron discharge device according to claim '7, wherein said energy extracting means comprises a cavity resonator located beyond said drift tube and having at least one aperture through which said beam path lies.

9. An electron discharge device according to claim 7, wherein said energy extracting means comprises a cavity resonator located beyond said drift tube having aligned apertures through which said beam path lies, and further comprising a collector in said path beyond said resonator.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,064,469 Haeff Dec. 15, 1936 2,239,421 Haefi Apr. 22, 1941 2,245,627 Varian June 17, 1941 2,259,690 Hansen et al Oct. 21, 1941 2,320,860 Fremlin June 1, 1943 2,406,370 Hansen et a1 Aug. 2'], 1946 2,578,434 Lindenblad Dec. 11, 1951 OTHER REFERENCES Article by A. V. Hollenberg, pp. 52-58, Bell System Tech. J our., for January 1949.

Article by A. V. Haeff, pp. 6-10, Proc. 1. R. E. for January 1949.

Article by L. S. Nergaard, pp. 585-596, RCA Rev. for December 1948. 

